It has been well known that certain peptides have physiological activities and their utilization has been studied extensively. Especially peptides bearing an acyl moiety at their N-terminal, viz., N-acyl peptides have shown interesting activity. Acid proteases, for example, have been reported to be specifically inhibited by N-acyl peptides, i.e., pepstatins, of the general formula: EQU R--Val--Val--X--Ala--X
wherein Val is L-valine, the X between Val and Ala is 4-amino-3-hydroxy-6-methylheptanoic acid and the other X is 4-amino-3-hydroxy-6-methylheptanoic acid or a salt or an ester thereof and Ala is L-alanine, and the carboxyl group of the X between Val and Ala being bound to the amino group of Ala to form a peptide bond and the amino group of the X between Val and Ala being bound to the carboxyl group of Val to form a peptide bond, the amino group of the other X being bound to the carboxyl group of Ala to form a peptide bond and the carboxyl group of the other X being free or esterified or bond to a cation to form a salt, and R is an acyl radical having carbon atoms of 2 to 8 or an acyl radical partially substituted by one or more hydroxyl groups or halogen atoms or a C-terminal of an esterfied carboxyl group, and the amino group of the Val adjacent to X being bound to the carboxyl group of the other Val to form a peptide bond and the carboxyl group of the R being bound to the amino group of the other Val to form an amide bond.
Some of these compounds are reportedly effective on gastric ulcers due to their anti-pepsin activity. Clinical studies to this effect have been published.
These N-acyl peptides are characterized by the presence of a novel amino acid, designated in the formula as X, which is thought to be closely related to their physiological activity. The length of the peptide chain and the character of the acyl group are also related to the activity of these peptides.
It is possible to presume that if a peptide bearing X but no acyl group, having a shorter peptide chain length and still maintaining the protease inhibitory activity can be obtained, it will exhibit more varied actions and be able to be widely utilized, because its physiochemical and biological properties such as solubility, absorption and distribution in various organs will be different.